This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Membrane transport proteins that utilize a 5-helix inverted repeat motif have recently emerged as the largest structural class of secondary active transporters. These membrane proteins are found throughout all kingdoms of life, and in mammals they are found in all tissues, where they are responsible for transporting small molecules such as amino acids and sugars across membranes. They use electrochemical gradients to concentrate these substrates using an alternating access mechanism originally outlined in the 1950s and 60s (1-3). How this mechanism works at the molecular level is only beginning to be understood as high-resolution structures are being determined. Here, we intend to use molecular dynamics simulations to investigate this process in the sodium-galactose co-transporter (SGLT) from the marine bacterium Vibrio parahaemolyticus, called vSGLT, whose structure was recently solved by our collaborators Drs. Jeff Abramson (UCLA) and Ernie Wright (UCLA) (4). Our proposed work will broadly address the basic biological phenomenon of energy transduction by secondary active transporters. Specifically, it will shed light on sugar metabolism and SGLT associated water transport, the later of which forms the basis of Oral Rehydration Therapy, which is estimated to have decreased childhood mortality due to severe diarrhea from 4.6 million in 1980 to 1.6 million in 2000 (5). We will carry out this work through two specific aims: Specific Aim 1. Characterization of sodium and galactose coupling in vSGLT. Specific Aim 2. Water and urea permeation through vSGLT.